cmd/dist: default to clang on OpenBSD
cmd/dist: default to clang on OpenBSD

OpenBSD ships with GCC 4.2, the last version of GCC that used GPLv2.
As that is quite old (current GCC version is GCC 9, GCC 4.2 was
released in 2007), default to clang.

Change-Id: Ib93e7b4f4f3ffb9e047e60ffca3696d26ab08aac
Reviewed-on: https://go-review.googlesource.com/c/go/+/193621
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

CAFxX/go,akutz/go,golang/go,nawawi/go,akutz/go,CAFxX/go,CAFxX/go,CAFxX/go,cloudflare/go,akutz/go,cloudflare/go,cloudflare/go,golang/go,CAFxX/go,cloudflare/go,nawawi/go,nawawi/go,golang/go,nawawi/go,nawawi/go,nawawi/go,cloudflare/go,akutz/go,golang/go,cloudflare/go,golang/go,akutz/go,nawawi/go,golang/go,CAFxX/go,akutz/go,golang/go,CAFxX/go,nawawi/go,cloudflare/go,cloudflare/go,golang/go,CAFxX/go,akutz/go,akutz/go